Đáp án:
$\begin{array}{l}
a)M = \left( {\sqrt {28} + \sqrt {63} - \sqrt 7 } \right):\sqrt 7 \\
= \left( {2\sqrt 7 + 3\sqrt 7 - \sqrt 7 } \right):\sqrt 7 \\
= 4\sqrt 7 :\sqrt 7 \\
= 4\\
b)N = 6\sqrt 2 + 2\sqrt {18} - \sqrt {50} + \sqrt {72} \\
= 6\sqrt 2 + 2.3\sqrt 2 - 5\sqrt 2 + 6\sqrt 2 \\
= 17\sqrt 2 \\
c)A = \sqrt {25a} - \sqrt {49a} + \sqrt {64a} \left( {a > 0} \right)\\
= 5\sqrt a - 7\sqrt a + 8\sqrt a \\
= 6\sqrt a \\
d)B = \sqrt {9 + 4\sqrt 5 } - \sqrt {9 - 4\sqrt 5 } \\
= \sqrt {5 + 2.2.\sqrt 5 + 2} - \sqrt {5 - 2.2.\sqrt 5 + 4} \\
= \sqrt {{{\left( {\sqrt 5 + 2} \right)}^2}} - \sqrt {{{\left( {\sqrt 5 - 2} \right)}^2}} \\
= \sqrt 5 + 2 - \left( {\sqrt 5 - 2} \right)\\
= 4\\
e)\\
P = \dfrac{{\sqrt 5 + \sqrt 2 }}{{\sqrt 5 - \sqrt 2 }} + \dfrac{{\sqrt 5 - \sqrt 2 }}{{\sqrt 5 + \sqrt 2 }} + \dfrac{{\sqrt {6 - 2\sqrt 5 } }}{{\sqrt 5 - 1}}\\
= \dfrac{{{{\left( {\sqrt 5 + \sqrt 2 } \right)}^2} + {{\left( {\sqrt 5 - \sqrt 2 } \right)}^2}}}{{\left( {\sqrt 5 - \sqrt 2 } \right)\left( {\sqrt 5 + \sqrt 2 } \right)}} + \dfrac{{\sqrt {{{\left( {\sqrt 5 - 1} \right)}^2}} }}{{\sqrt 5 - 1}}\\
= \dfrac{{5 + 2\sqrt {10} + 2 + 5 - 2\sqrt {10} + 2}}{{5 - 3}} + \dfrac{{\sqrt 5 - 1}}{{\sqrt 5 - 1}}\\
= \dfrac{{14}}{2} + 1\\
= 7 + 1\\
= 8
\end{array}$
